1. Technical Field of the Invention
The present invention relates to avalanche photodiodes and, in particular, to arrays of avalanche photodiodes.
2. Description of Related Art
An avalanche photodiode (APD) is a semiconductor photosensor device capable of light detection. It is especially useful in low, weak or reduced light applications as the avalanche phenomenon utilized by the device provides for a significant degree of amplification. Avalanche occurs when carriers generated by photon induced free carrier generation in a light receiving area of the device are introduced into a high electric field area formed in a highly reverse biased (near breakdown voltage) semiconductor pn junction. The introduced carriers collide with neutral atoms to release other carriers by impact ionization. This collision process is then repeated and repeated in an avalanche fashion to effectively amplify the limited number of carriers that were initially produced by photon induced free carrier generation. It is well known that the amplification factor (i.e., gain or responsivity) of an avalanche photodiode is heavily dependent on the applied reverse bias.
FIG. 1 illustrates in cross-section a conventional semiconductor avalanche photodiode device 10. The device includes a p silicon substrate layer 12. On that substrate layer 12 is formed an intrinsic (p−) layer 14 which constitutes the light (hv) receiving layer and which may, for example, comprise an epitaxial growth layer. A p layer 16 is then formed on the intrinsic layer 14 through, for example, boron diffusion. An n+ layer 18 is then formed in the p layer 16 through, for example, phosphorus diffusion. An electrode (perhaps aluminum) 20 is provided to contact the n+ layer 18. Another electrode (also perhaps aluminum) 22 is provided for the p silicon substrate layer 12. The electrodes 20 and 22 comprise the contact terminals for the photodiode device 10, and it is between these electrodes that a reverse bias voltage is applied. Individual photodiode devices 10 may be assembled together in an n×m array format to form an avalanche photodiode array 24 as illustrated in FIG. 2 wherein each of the devices share a common, single monolithic substrate.
A limiting factor in the use of avalanche photodiode devices is the careful control that must be exercised over the electric field generated by the applied reverse bias. The gain of the avalanche photodiode device is a function of that applied electric field. It is important that the electric field be established at an optimal level so that the device gain is neither too high nor too low. Unfortunately, the device gain value cannot be easily measured. It is, however, recognized by those skilled in the art that the responsivity of the avalanche photodiode device can be measured and that measuring responsivity substitutes as a proxy for measuring gain. Thus, by effectuating some measure of control over device responsivity, one can to some degree control gain. A primary reason one desires to control gain is to minimize system noise. In an optical receiver using an avalanche photodiode, there is an certain optimal gain where overall system noise for the receiver is minimized. If the gain is too low, the noise of the preamplifier used to convert the avalanche photodiode output signal to a useable level will dominate. Conversely, if the gain is too high, the output of the avalanche photodiode device may become saturated by either dark current or incident light, or become dominated by excess noise generation.